Human diseases that enhance cancer predisposition are associated with increased chromosome rearrangements. Yeast homologs have been found for a number of these disease genes and mutation of the yeast gene shown to cause hyper-recombination. Study of such yeast hyper- recombination mutants will lend insight to the disease processes and cancer etiology in general. The S. cerevisiae Sgs1 DNA helicase is highly homologous to the proteins which are mutated in the Wenner, Bloom and Rothmund-Thomson cancer predisposition syndromes. Sgs1 mutants were identified as suppressors of the slow growth of top3 DNA topoisomerase mutants. top3 mutants exhibit pleiotropic defects including hyper-recombination. The sgsl mutant phenotype is reminiscent of top3, albeit generally less severe. SGS1 epistasis to TOPS indicates that they function in the same pathway. In addition to Top3, yeast also contain the Top1 and Top2 topoisomerases. In pairwise combinations, the three topoisomerases exhibit synergistc defects indicating functional overlaps. Sgs1 physically interacts with Top2 and Top3. sgsl in combination with either topl or top2 mutation exhibits synergistic defects. The functional overlaps/antagonisms among Sgs1/Top3 and the other topoisomerases are not understood. The DNA substrates upon which Sgs1 and Top3 act in vivo are not known. Interestingly, Sgs1/Top3 interaction is conserved in humans. The knowledge of when and where Sgs1 and Top3 act during DNA replication/recombination in yeast can lend important insight to the human disease process and cancer etiology. The goal of this proposal is to investigate the functions of Sgs1 and Top3 in S. cerevisiae using combined genetic, biochemical, and cell biological approaches.